The present invention relates to an alloy for anti-corrosive coatings or surface coatings which are resistant up to 1100xc2x0 C.
Such surface coatings can be obtained by the following thermal spraying methods: plasma spraying in vacuo, plasma spraying in air, HVOF (High-Velocity Oxygen Fuel) or high-velocity oxygen flame.
In the field of metallurgy, and in particular in the field of alloys used as coatings for gas turbines against corrosion caused by sulpho-oxidation, an alloy based on cobalt and also containing chromium, aluminium and yttrium in the following proportions:
is already known.
Said alloy is resistant to corrosion and guarantees a service life of 1000 hours if used for the coating of components which operate in the presence of sodium-, vanadium-, sulphlur- and chlorine-containing hydrocarbon combustion products.
A first disadvantage is that this alloy does not guarantee sufficient corrosion resistance if employed at temperatures above 700xc2x0 C.
Furthermore, if said alloy is applied by a thermal spraying method (APS, VPS, HVOF), oxidation of yttrium and aluminium occurs, thus resulting in the loss of the working property of the coating.
A second alloy based on cobalt and containing chromium, aluminium, yttrium and silicon in the following proportions:
is also known
Said second alloy is used for coating components with a superalloy material. The presence of silicon in the coating matrix provides resistance against the formation of oxides and sulphides on the surface layer.
However, this second alloy has a low yttrium content, which is insufficient for ensuring corrosion resistance in the presence of products containing substantial amounts of sulphur, chlorine and vanadium.
A third alloy is known from U.S. Pat. No. 4,585,481, which describes an alloy based on cobalt and containing chromium, aluminium, yttrium and silicon in the following proportions:
However, this alloy results in a reduction of the plasticity of the coating, making it impossible to use it for the coating of apparatuses or objects which must withstand substantial changes in temperature.
A further disadvantage is that, during application of the coating or surface coating by the plasma method, oxidation of yttrium and silicon may take place.
This third alloy is unable to guarantee the corrosion resistance required in the presence of combustion products containing substantial amounts of sulphur, chlorine and vanadium.
The alloys mentioned above can usually be used at temperatures not exceeding 900xc2x0 C., since all of them are based on cobalt.
RU-A-2051196 teaches an alloy for corrosion-resistant coatings or surface coatings, containing of cobalt, chromium, aluminium, yttrium, silicon, and a metal from the second main group, together with the corresponding oxide, in the following proportions:
This alloy resists only up to 800xc2x0 C.